1. Field of the Invention
The present invention relates generally to recovery of tungsten in purified form from tungsten-bearing material. More specifically the invention relates to solvent extraction processes in which tungsten values are extracted from an aqueous phase, such as a solution of sodium tungstate, into an organic phase. The tungsten values can subsequently be stripped into an ammoniacal aqueous phase for eventual recovery of an ammonium tungstate product.
Solvent extraction is customarily employed in the recovery of tungsten to separate tungsten values from contaminants accompanying the tungsten in solution in the aqueous phase. These contaminants include sodium ion and sulfate ion, and also include silica, fluorine, calcium, and magnesium. The tungsten-bearing aqueous phase is intimately mixed with an organic phase typically comprising at least one extractant for the tungsten values and one or more other liquid organic compounds. The tungsten values are extracted into the organic phase, and the mixture of organic and aqueous phases is allowed to settle so that the two phases can be recovered separately. The steps of mixing and settling can be repeated several times to improve the overall degree of extraction of tungsten values.
Several problems are commonly associated with the extraction of tungsten values from an aqueous phase into an organic phase, and the present invention effectively deals with these problems. One problem is the contamination of the organic phase with impurities from the aqueous phase. In particular, sodium ion and oxidized sulfur ions (such as sulfate and bisulfate) can transfer into the organic phase, in which case they are carried with the tungsten into the final product and contaminate that product. Accordingly, it is desirable to minimize transfer of sodium into the organic phase, and to minimize the amounts of sulfate and bisulfate which are present in the final tungsten product. These ions can be transferred by extraction or by physical entrainment of the aqueous phase in the organic phase.
Another problem encountered in solvent extraction of tungsten values is the formation of a third phase. The third phase can comprise liquid, solids, or a mixture of liquid and solid. The third phase is formed when the tungsten content extracted into the organic phase exceeds a moderate level. The third phase builds up, so that process equipment must periodically be shut down to remove it, and it represents lost extraction efficiency. This problem can be caused by the excessive transfer of impurities from the aqueous phase into the organic phase, or by the choice of constituents used in the organic phase. This drawback can require the operator to tolerate low tungsten contents in the organic phase so as to avoid formation of the third phase.
A related problem is the slow rate at which the aqueous and organic phases separate in the settling stage. Stated another way, the mixture of aqueous and organic phases fed to a settler should separate quickly into two discrete phases, but in actual practice relatively long periods of time can be necessary to separate the two phases to a satisfactory degree.
Thus, there is a need for a process for extracting tungsten values from an aqueous phase into an organic phase, wherein substantial amounts of tungsten and only minor amounts, if any, of impurities are extracted into the organic phase, and wherein a mixture of the aqueous and organic phases settles rapidly to form only two discrete phases.
2. Description of the Prior Art
U.S. Pat. No. 4,175,109 to Kim describes a solvent extraction process for tungsten in which a specific organic extractant is employed to extract tungsten from an aqueous phase having an initial pH of 2.0. The extractant comprises 6 to 10 vol. % of tri-isooctylamine, optionally including other tertiary amines, in an alkyl benzene solvent having a molecular weight of 120, 134, or 148. This patent does not disclose the effect of the pH of the aqueous feed and of the raffinate on the advantages discussed herein that are afforded by the present invention.
U.S. Pat. No. 4,092,400 to Zbranek discloses a process in which tungsten values are extracted from an aqueous phase into an organic phase which comprises an amine extractant, such as a secondary amine having substituents each 8 to 22 carbon atoms in length, and a modifier such as isodecanol, dissolved in kerosene. In the disclosed process, the pH of the aqueous phase is adjusted to 1 to 1.8, and maintained at 1 to 1.8; pH values above 1.8 are said to cause emulsion problems, substantially slower phase separation, and incomplete tungsten extraction. The patentee does not disclose or suggest the advantages applicants have discovered of maintaining the initial pH of the aqueous phase, and the pH of the aqueous raffinate, within the specific ranges disclosed and claimed herein.
U.S. Pat. No. 3,158,438 to Kurtak also discloses extracting tungsten values from an aqueous phase into an organic phase which comprises an organic amine, such as ditridecylamine, dissolved in an aromatic solvent or kerosene with a long-chain aliphatic alcohol modifier. However, use of ditridecylamine and several other amines is said to provide less than 90% extraction of tungsten into the organic phase. The aqueous phase can have a pH of below 3, such as 2.1 to 2.3, as a result of the acidification in a previous treatment step for removal of molybdenum. There is no recognition of the advantageous effects of providing both the raffinate pH and the aqueous feed pH within the specific ranges applicants have discovered.
The authors of an article in "Soviet Journal of Non-Ferrous Metals," 13 (3) (March, 1972) pp. 42-44, described a series of tests in which a pure aqueous sodium tungstate solution was extracted with an equal volume of an organic phase comprising 10 vol. % of a mixture of tertiary amines having substituents with 7 to 9 carbon atoms, and 20 vol. % octyl alcohol, dissolved in kerosene, at about 20.degree. C. At an "equilibrium pH" of 2.5 to 4.5, over 99% of the tungsten was extracted from an aqueous solution having an initial tungsten content of 50.5 gpl WO.sub.3. At an "equilibrium pH" of 3.0 to 4.2, over 97.6% of the tungsten was extracted from an aqueous solution having an initial tungsten content of 89.35 gpl WO.sub.3. There is no disclosure, however, of the effect of the aqueous feed pH or the raffinate pH on such features as sodium ion or sulfate ion transfer into the organic phase, the rate of disengagement of the organic and aqueous phases, or the formation of a third solid or liquid phase. In addition, there is no recognition of the particular pH ranges disclosed and claimed herein for the aqueous feed and the aqueous raffinate to be used in an extraction system, which employs ditridecylamine as opposed to tertiary amines; it is noted that the ditridecylamine employed in the process of the present invention provides significantly superior results.